supplementary materials

Poly[di-9-citrato-tetra­sodiumzinc]

In the title compound, [Na4Zn(C6H5O7)2]n, the ZnII ion lies on an inversion center and is coordinated by six O atoms from two citrate ligands, forming a distorted octa­hedral geometry. There are two crystallographically independent Na+ cations in the asymmetric unit. One Na+ cation exhibits a distorted square-pyramidal geometry defined by five O atoms from four citrate ligands. The other Na+ cation is surrounded by six O atoms from five citrate ligands in a distorted octa­hedral geometry. The Na+ cations are bridged by citrate carboxyl­ate groups, forming a layer parallel to (100). The layers are further assembled into a three-dimensional network with the [Zn(citrate)2]4- building units as `pillars'; O-HO hydrogen bonds also stabilize the structure.

Citric acid has been widely used for the construction of coordination polymers
due to their diverse coordination modes (Liu et al., 2012).
Here, we report a new
three-dimensional coordination polymer, [Na4Zn(C6H5O7)2]n,
based on citric acid.

As shown in Fig. 1, the asymmetric unit of the title compound consists of
half a ZnII ion, two Na+ cations and a citrate anion. The ZnII ion
lies on a crystallographic inversion center and is coordinated by six O atoms
from two different citrate ligands, forming a distorted octahedral geometry.
Three O atoms of each citrate ligand are bonded to the ZnII ion,
one of which is the hydroxy O atom and the other two are from different
carboxylate groups. Thus, two citrate ligands and one ZnII ion form a
[Zn(C6H5O7)2]4- building unit. This unit bridges sixteen Na+
cations (Fig. 2). Na1 exhibits a distorted square-pyramidal geometry,
defined by five O atoms from four different citrate ligands. Na2 is
surrounded by six O atoms from five different citrate ligands, building a
distorted octahedral geometry. The Na+ cations are bridged by carboxylate
groups from the citrate ligands into a two-dimensional layer parallel to (100)
(Fig. 3). The layers are further assembled into a three-dimensional network
through [Zn(C6H5O7)2]4- building units as 'pillars' (Fig. 4).

A mixture of citric acid (0.2 mmol), NaOH (0.2 mmol) and zinc nitrate
hexahydrate (0.1 mmol) was dissolved in DMAC/H2O solvent (5 ml, v/v = 1:4)
(DMAC = N,N'-dimethylacetamide) and placed in a capped
vial (10 ml), which was heated to 363 K for three days and then cooled to room
temperature. The crystals obtained were washed with water and dried in air.

C-bound H atoms were placed at calculated positions and refined as
riding atoms, with C—H = 0.97 Å and with
Uiso(H) = 1.2Ueq(C).
The hydroxy H atom was located in a difference map and refined isotropically.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
σ(F2) is used only for calculating R-factors(gt) etc.
and is not relevant to the choice of reflections for refinement.
R-factors based on F2 are statistically about twice as large
as those based on F, and R- factors based on ALL data will be
even larger.